With the advancement of information-intensive society, a variety of displays have been developed. As one of the typical examples of thin film type light emitting elements for the use in such displays, organic electroluminescent (or EL) emitting devices are given.
FIG. 7 schematically shows a structure of an organic electroluminescent emitting body in its sectional view in which a light-emitting element (15) is provided on a light-transmitting substrate (1). Light emitted from the light-emitting element (15) passes through the substrate (1) and ejects out of an exposed surface of the substrate (1) so as to be taken out to the external. As shown by the arrows in FIG. 7, the light which enters the interface between the surface of the substrate (1) and ambient air at a small incident angle ejects out of the exposed surface of the substrate (1), while most of light reflects by the interface between the exposed surface of the substrate (1) and the ambient air, and are guided within the substrate (1) in directions toward edge portions of the substrate. As a result, it is practically only about 20% of the emitted light that can go out of the exposed surface of the substrate (1). This phenomenon is one of the main reasons why a light discharge efficiency (or light external efficiency or coupling-out efficiency) of the thin film type light-emitting elements is lowered.
To overcome this problem, various devices for avoiding such a phenomenon to thereby improve the light discharge efficiency from the exposed surface of the substrate (1) are proposed. For example, a light-diffusing layer (16) is formed on an exposed surface of a substrate (1) as shown in FIG. 8 which schematically shows a sectional view thereof by finely processing. According to this structure, light is diffused by the light-diffusing layer (16) of the substrate (1) so as to reduce the light guide within the substrate (1) so as to improve the light discharge efficiency from the exposed surface of the substrate (1) to its external. For such fine processing, micro-lens processing, diffusion processing and the like may be used.
However, in the case of FIG. 8, a thickness of the substrate (1) is generally in the order of millimeter, and therefore, it is a few times that light being guided strikes the light-diffusing layer (16). Thus, the effect of improving the light discharge efficiency by controlling light guiding is insufficient. Further, there arises another problem in that, since lights are diffused by the light-diffusing layer (16), lights are mixed which makes it hard to obtain sharp contrast when it is needed that emitted lights should be recognized as an image as in the case of a display and the like.
In other embodiment, there is provided a lower refractive index layer (17) having a lower refractive index (for example, 1.3 or lower) than that of a substrate (1) between the substrate (1) and a light-emitting element (15) as schematically shown in FIG. 9 in a sectional view. With this arrangement, light is refracted through the interface between the lower refractive index layer (17) and the substrate (1) to thereby reduce an incident angle of the light at interface between the exposed surface of the substrate (1) and ambient air. As a result, a quantity of light is decreased which is reflected by the interface between the exposed surface of the substrate (1) and the air, and consequently, the light guide within the substrate (1) is suppressed, and the light discharge efficiency from the surface of the substrate (1) to the external is improved.
In case of the embodiment shown in FIG. 9, the insertion of the lower refractive index layer (17) is able to substantially eliminate the light guide within the substrate (1), so that the light discharge efficiency can be improved. However, there is a problem in the case where a thickness of the light-emitting element (15) is large which has a refractive index higher than that of the lower refractive index layer (17). That is, a quantity of light becomes larger which is reflected by the interface between the light-emitting element (15) and the lower refractive index layer (17), so that there is a danger that the light guide is likely to be increased in the light-emitting element (15), which requires to deliberately select the thickness of the light-emitting element (15).
As described above, it is hard to improve the light discharge efficiency of the thin film type light-emitting elements in the case where emitted light is taken out to the external (into the ambience), and therefore, the improvement of the light discharge efficiency becomes a subject to be solved.
In general, the light discharge efficiency η where light which is generated in a light emitting device comprising a surface light-emitting element is taken out is determined by a critical angle θc upon total reflection where light is ejected from a medium having a refractive index of n into an air having a refractive index of 1.0, based on the classical optics rules.
The critical angle θc can be calculated by the following equation (1), according to the rule of refraction:sin θc=1/n  (1)
The discharge efficiency η is determined by the following equation (2), from a ratio of a quantity of light which passes through a medium having a refractive index of n into an air to a total quantity of light generated (i.e. the sum of a quantity of light which is totally reflected by the interface between the medium and the air and a quantity of light which goes into the air):η=1−(n2−1)1/2/n  (2)
It is noted that when the refractive index n of the medium is higher than 1.5, the following approximate equation (3) may be used. However, when the refractive index n of the medium is very close to 1.00, it is needed to use the above equation (2).η=1/(2n2)  (3)
In the thin film type light emitting device such as an electroluminescent (EL) element or the like, a thickness of the part of the surface light-emitting element is smaller than a wavelength of the light, and therefore, the discharge efficiency η is mainly controlled by the refractive index of the substrate. The refractive index n of glass, a plastic film or the like used as the substrate is generally about 1.5 to about 1.6. Therefore, the discharge efficiency η is about 0.2 (about 20%) according to the equation (3). In other words, the rest of about 80% is lost as guided light due to the total reflection at the interface between the substrate and the air.
Typical examples of the thin film type light emitting devices include an organic EL element and a photoluminescent (or PL) light-emitting element comprising a PL light-emitting layer is also similar. The PL light-emitting element has a structure in which a PL light-emitting layer is laminated on a substrate. When the PL light-emitting layer of this element is irradiated with light such UV or the like, the PL light-emitting layer emits light, and the light ejects from the substrate. The light discharge efficiency η of this element is also low, and most of light is as guided light because, in general, the light-emitting device is formed on the substrate similarly to the above case.
Under the foregoing circumstances, Japanese Patent Kokai Publication JP-A-2001-202827 discloses a technique of lessening light loss as guided light within a substrate by forming a light-emitting element on a surface layer of a substrate which surface layer has a low refractive index. According to this publication, the light discharge efficiency is improved by forming a thin film light-emitting element on a thin film with the lower refractive index. In a light-emitting element having a thickness smaller than a wavelength of the light, the light guiding within its light-emitting layer is restricted, and therefore, a quantity of light to be able to be radiated through a surface of the light-emitting layer is increased. Specifically, for example “Applied Physics Letters” (vol. 78, No. 13, p. 1927) describes that a phenomenon where the light guiding is markedly restricted when a thickness of a light-emitting layer (a thickness of a transparent electrically conductive film is included in the case of an organic EL) is about 200 nm or less.
It is apparent from the above that the light discharge efficiency becomes higher when a light emitting device is produced by forming a light-emitting layer on a substrate which has a surface layer with a lower refractive index. In addition, it is advantageous for the formation of a light emitting device that a surface layer as a thin film which satisfies the following relationship (4) is formed on a substrate:n2<n1  (4)(wherein n1 is a refractive index of the substrate; and n2 is a refractive index of the surface layer having a lower refractive index previously formed on the surface of the substrate on the side where the light-emitting layer is to be formed). Glass and a plastic film are generally used as the substrate, and they have a refractive index of about 1.5 to about 1.6.
The number of combination of a substrate with a thin film to be formed on the substrate which satisfies the relationship of equation (4) can be said to be infinite. Practically, when the refractive index of a light-emitting layer and conditions for forming the light-emitting layer (e.g., temperature, a process for the formation, etc.) are appropriately selected, the light discharge efficiency of the light emitting device is improved as compared with a light emitting device comprising a substrate without any thin film on a surface thereof. Japanese Patent Kokai Publication JP-A-2001-202827 proposes that a thin film with a refractive index of 1.003 to 1.300 is provided on a surface of the conventional substrate, and specifically that a porous thin film such as silica aerogel is formed thereon. However, strength of this thin film with a low refractive index is not always sufficient, since the film is porous.